Copolymers of monoethylenically unsaturated dicarboxylic acids, monoethylenically unsaturated monocarboxylic acids and carboxyl-free monoethylenically unsaturated monomers are useful as antiscalants, dispersants, incrustation inhibitors and builders in detergents, deflocculants and drilling fluid aids. Several techniques employing aqueous polymerization systems have been previously described in the preparation of these copolymers.
Initially, high molecular weight polymers of this type were produced using a redox system, while the more desirable low molecular weight polymers were produced using a chain transfer agent such as a mercaptan. However, mercaptans are undesirable because of both their expense and unpleasant odor. U.S. Pat. No. 4,314,044 ('044) attempted to solve this problem of using mercaptans by describing an aqueous process for the preparation of low molecular weight water-soluble copolymers of acrylic and methacrylic acid with unsaturated dicarboxylic acids such as itaconic acid, fumaric acid and maleic acid. The '044 patent disclosed the use of a catalytic system comprising an initiator and a metal salt in a molar ratio of initiator to metal ion of from about 10:1 to about 150:1 and the initiator being present in an amount of from 0.5 to about 35 weight percent based on the weight of the monomers present.
The process of the '044 patent was modified by U.S. Pat. No. 4,659,793 ('793) which describes an aqueous batch process for the preparation of copolymers of monoethylenically unsaturated aliphatic dicarboxylic acids with .alpha.,.beta.-ethylenically unsaturated monomers. By using one of various metal ions at low levels, such as 1 to 250 ppm, the residual monomer content was claimed to be reduced to 0.5% or less based on the total weight of the dried copolymers. The '793 patent describes a copolymer containing as copolymerized monomer units of from 25 to 55% by weight of the dicarboxylic acid and from 45 to 75% by weight of the ethylenically unsaturated monomers and said copolymers are claimed to have a weight average molecular weight of from about 2,000 to 1,000,000. However, the lowest level of the dicarboxylic acid exemplified in the '793 patent is 30%, yielding a weight average molecular weight of from 25,000 to 40,000. The metal ions disclosed as suitable for use in the process of the '793 patent are Zn++, Co+++, Co++, Cu++, Mo++, Fe+++, Fe++, Cr+++, Ni++, Ce++++ and Ce++. No metal ion is shown to have any advantage over any of the other metal ions and only Fe++ is shown in the specific examples of the patent.
Another process for the synthesis of these polymers is a continuous cascade process disclosed in U.S. Pat. No. 4,725,655. Described in this patent is the copolymerization of monoethylenically unsaturated dicarboxylic acids in concentrations of from 10 to 60% by weight of the monomers, monoethylenically unsaturated monocarboxylic acids in concentrations of 40 to 90% by weight of the monomers and optionally carboxyl-free monomers in concentration of not more than 20% by weight of the monomers. This reaction is carried out in the presence of from 0.5 to 6% by weight of a water-soluble, free radical forming initiator. Multiple reactors are required for this procedure with the preferred number being at least three. This process does not claim the use of any metal salt.
Copending U.S. patent application Ser. No. 353,376 ('376) filed on May 17, 1989, now abandoned commonly assigned to the same assignee as the present invention, also describes a method of preparation of copolymers of monoethylenically unsaturated aliphatic dicarboxylic acids with .alpha.,.beta.-ethylenically unsaturated monomers. The '376 application describes a continuous or batch process for producing copolymers of more consistent composition by the simultaneous addition of the monomers to the reaction vessel. The monoethylenically unsaturated dicarboxylic acid concentration is from 5 to 65%, preferably from 10 to 60% by weight of the total monomer concentration and the monoethylenically unsaturated monocarboxylic acid concentration is from 35 to 95%, preferably from 40 to 90% by weight of the total monomer concentration. This procedure also makes use of a metal salt in concentrations from 1 to 200 ppm, preferably from 1 to 25 ppm based on the total weight of the monomers. The metal ions include water soluble salts of cobalt, iron, copper, nickel, manganese, zirconium and zinc. Again, no metal ion is shown to have any advantage over any of the other metal ions in controlling the molecular weight of the copolymer.
Currently, there is no process to control the molecular weight and degree of branching during the preparation of copolymers containing low levels of aliphatic monoethylenically unsaturated dicarboxylic acids of from about 3 to 25%. When any of the previously described techniques are used to synthesize copolymers at the range of the current invention, the copolymer tends to branch dramatically during synthesis and the molecular weight builds. In most circumstances this causes the reaction solution to increase viscosity and eventually gel. When copolymers have been synthesized under these prior art techniques, the molecular weights have generally been too high and therefore, the copolymers have been undesirable.
It is therefore a principal object of this invention to furnish novel water soluble, low molecular weight copolymers containing low levels of monoethylenically unsaturated dicarboxylic acid units, such as maleic acid, and high levels of the monoethylenically unsaturated monocarboxylic acid units, such as acrylic acid. Detergent and water treatment formulations containing these novel copolymers are also within the invention.
It has now been discovered that polymers containing low levels of monoethylenically unsaturated aliphatic dicarboxylic acids can be successfully synthesized and the molecular weight controlled at a desirable level by using low levels of copper salt as polymerization moderator. Surprisingly, copper salt has been found to be unique in its properties among the common metal salts disclosed in the previous art.